In vehicles having an electronically controllable air spring system (ECAS), it is possible using air springs that are arranged for each individual wheel to realize that a chassis of the vehicle is constantly height adjusted according to a desired set specification during an arbitrary driving situation in order to improve the driving comfort and to perform level changes as a result of a changed load. For this purpose, a specific bellows volume for each individual wheel is set in the air springs or bellows of the air springs, which are arranged by way of example in pairs on an axle, in order to raise or lower the chassis in the region of the respective air spring and consequently to ensure that the chassis is height adjusted in a specific manner with respect to the vehicle axles or the terrain on which the vehicle is moving.
The current prevailing bellows-volume in the bellows may be characterized in this case by means of an actual height value that indicates the height of the chassis above an axle-fixed reference point. The bellows-volume is set in this case automatically using a closed-loop control algorithm, which is implemented on a control module of the air spring system and which in dependence upon the actual height value that characterizes the prevailing orientation of the vehicle at the respective air spring sets a desired height value by means of changing the bellows-volume, by way of example by means of providing compressed air at a specific air pressure from a pressure source that is connected to the air springs, by way of example via a compressor or from a reservoir in order to re-instate the orientation of the vehicle in accordance with the desired specification.
This is encumbered with the disadvantage that when the vehicle is at a standstill, if the vehicle is in the process of being loaded or unloaded, the air spring system continues within terms of adjusting the level to re-instate the desired specification on the basis of the changed weight and the associated deviation from the desired specification. If the vehicle is being loaded by way of example via a forklift truck, which so as to load the vehicle is lying with its fork on a loading surface of the vehicle and is pressing said loading surface downwards by means of the additional mass and intrinsic weight of the forks, any raising of the loading surface as a result of the triggered level adjustment may cause the forklift truck to become damaged since the loading surface presses against the forks of the forklift truck. In contrast, if the vehicle is being unloaded, as the load is being raised using the forklift truck, it is possible that the loading surface is lowered as a result of the level adjustment in order to compensate for the missing weight. This may irritate the driver of the forklift truck if he is accustomed to conventional vehicles that have steel springs. Consequently, it is possible aside from the driving operation of the vehicle for situations to occur in which the level adjustment via the air spring system is obstructive or unusual.
In order to avoid this, methods are known that switch off the air spring system during a detected loading and/or unloading procedure, in particular using a forklift truck. DE 195 39 887 B4 describes for this purpose that, in the event that a loading and/or unloading procedure is established, closed-loop control procedures of the air spring system are suppressed or minimized in different embodiment variants. Accordingly, it is possible to provide that, prior to a closed-loop control intervention being performed in response to a detected level change, a time delay is selectively adapted, in particular extended, if a loading or unloading procedure has been detected, with the result that when the vehicle is being loaded and/or unloaded the actual air spring pressure in the air springs no longer changes immediately. In accordance with a further variant, it is possible to increase a so-called closed-loop control deadband if a loading or unloading procedure is detected, wherein the closed-loop control deadband indicates above which point of deviation of the actual height value from a desired height value a level control is to be performed. If in the presence of a loading or unloading procedure the control deadband is accordingly adapted in such a manner that a control procedure is only performed if a load value that is to be expected by the forklift truck is exceeded, it is possible to avoid a control procedure during a loading and/or unloading procedure. It is provided in accordance with a third variant that a level control is only performed if, when the vehicle is at a standstill, a level sensor signal is no longer indicating changes in the orientation of the chassis. Accordingly, the level of the chassis is not adjusted as long as the loading surface is moved during a loading or unloading procedure and moves according to which procedure is being performed. This is encumbered with the disadvantage that it is necessary for the loading and unloading procedure to be specified manually and it is not possible to detect said procedure automatically.
Furthermore, U.S. 2013/338876 A1 describes installing low-pass filters in a control module of the air spring system, wherein and said low-pass filters filter a control signal that is generally a desired height value, which is to be controlled via the air springs in a closed-loop manner, and in the event that a limit value between different filtered control signals is exceeded it is concluded therefrom that the vehicle is currently being loaded or unloaded. In the event that a loading or unloading procedure is detected, a further closed-loop control intervention via the control module of the air spring system is prevented. This is encumbered with the disadvantage that it is extremely complex to filter and consequently to process the signals.